Make HapiReader consistent with LaTiS 3 changes

[lindholc]

The DatasetReader interface has changed since HapiReader was written.

[lindholc]

@dlindhol, I'm confused about what DatasetReader does now.

DatasetReader now has an abstract model member that needs to be defined to make a non-abstract implementation like HapiReader. However, HapiReader currently computes the model from the given URI.

Does that mean DatasetReaders need to be given the model rather than compute it themselves? So HapiReader would be constructed with a model? Is HapiReader an abstract class that does most of what's necessary for getting a dataset from a HAPI server but needs to have subclasses (anonymous or not) for specific datasets?

[dlindhol]

We can remove the model from the DatasetReader. I added that to support the ReaderOperation used only in Hylatis, so far. We can find another way.